[unreadable] Opioid and adrenergic agonists produce analgesia synergistically at G protein-coupled receptors (GPCR). Opioid tolerance and neuropathic pain both involve neuronal plasticity that may decrease this synergistic relationship. The proposed research will 1) determine how synergistic interactions occur between opioid receptor (OR) and alpha-2 adrenergic receptor (AR) agonists, 2) compare the mechanisms underlying GPCR-mediated analgesic synergism under normal conditions and in states of neuronal plasticity, and 3) examine the functional relationships between OR and AR transmitters. We will test the following hypotheses: Hypothesis 1A - Synergistically acting receptor pairs sharing localization couple through different intracellular pathways; Hypothesis 1B - Synergistically interacting receptor pairs coupling through the same intracellular pathways reside in different locations; Hypothesis 2 - Persistent alterations in receptor distribution or coupling driven by neural plasticity impact potency or efficacy of analgesic agonists by changing interactions between receptor subtypes. Behavioral studies will examine the mechanism of spinal analgesic synergy under normal, opioid tolerant, and neuropathic conditions. Immunohistochemical localization of OR/AR and quantification of transmitter release from spinal cord slices will define the anatomical substrates for the functional interactions observed. Electrophysiology of dorsal horn neurons will examine the cellular mechanisms of drug action and interaction at the single cell level. The information gained from these studies may identify new therapeutic analgesic combinations and combination receptor targets leading toward reduction of dependence liabilities of analgesic treatments. [unreadable] [unreadable] [unreadable]